Prevent multiple instances of cromo from running.

DBus::Connection::request_name() does not indicate whether it
successfully acquires the requested DBus name or not. This CL changes
cromo to use newly introduced DBus::Connection::acquire_name()
(CL:195002) to ensure that cromo can successfully acquire its DBus name.

BUG=chromium:364585
TEST=Tested the following:
1. `USE=cellular FEATURES=test emerge-$BOARD platform2`
2. Manually run the following commands in a root shell on the DUT:
   - `stop cromo` and then `start cromo` works
   - `restart cromo` works
   - `cromo` fails to acquire the DBus name when another instance of
     cromo is already running.

Change-Id: Ie294521a864d3247a662340c4737ec3d1f13f1cd
Reviewed-on: https://chromium-review.googlesource.com/195482
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
2 files changed